1. Field of the Invention
The present invention relates to a target searching device, a target searching program, and a target searching method for searching a target by using passive-phase conjugate processing.
2. Description of the Related Art
In the field of underwater acoustics, there have been more researches conducted on signal time-reversals (Non-Patent Document 1: S. Kim, W. A. Kuperman, W. S. Hodgkiss, H. C. Song, G. F. Edelmann, and T. Akal, “Robust time reversal focusing in the ocean,” J. Acoust. Soc. Am. 114, 145-157(2003); Non-Patent Document 2: S. C. Walker, P. Roux, and W. A. Kuperman, “Focal depth shifting of a time reversal mirror in a rang-independent waveguide,” J. Acoust. Soc. Am. 118, 1341-1347(2005); and Non-Patent Document 3: S. C. Walker, W. A. Kuperman, and P. roux, “Active waveguide Green's function estimation with application to time-reversal focusing without a probe source in a rang-indepent waveguide,” J. Acoust. Soc. Am. 120, 2755-2763(2006)). When searching a target in shallow water, sound waves transmitted into the water are reflected at the sea surface or the seabed, thereby generating reflected waves. This makes it difficult to search the target in some cases.
Recently, a searching method which applies phase conjugate processing on reflected waves from a target has drawn attentions (Non-Patent Document 4: G. Mcntaldo, M. Tanter, and M. Fink, “Revisiting iterative time reversal processing: Application to detection of multiple targets” J. Acoust. Soc. Am. 115, 776-784(2004); Non-Patent Document 5: C. Prada, S. Manneville, D. Spoliansky, and M. Fink, “Decomposition of the time reversal operator: Detection and selective focusing on two scatterers,” J. Acoust. Soc. Am. 99, 2067-2076(1996); Non-Patent Document 6: N. Mordant, C. Prada, and M. Fink, “Highly resolved detection and selective focusing in a waveguide using the D. O. R. T. method,” J. Acoust. Soc. Am. 105, 2634-2642(1999); Non-Patent Document 7: D. H. chambers, “Analysis of the time-reversal operator for scatterers of finite size,” J. Acoust. Soc. Am. 112, 411-419(2002); and Non-Patent Document 8: D. H. chambers, A. K. Gautesen, “Time reversal for a single spherical scatterer,” J. Acoust. Soc. Am. 109, 2616-2624(2001)). These searching methods use backward scattering waves which scatter backward from the target. Now, there is considered a case where a sound wave makes incident on a target. When the sound wave makes incident on the target, a part of the sound wave is scattered at the target and reversed towards a sound source side as a scattered wave. Such scattered waves are called backward scattering waves. Meanwhile, a part of the sound wave is scattered forward from the target. Such scattered waves are called forward scattering waves.
The searching methods disclosed in Non-Patent Documents 4-8 use the backward scattering waves which scatter backward from the target. However, in cases where the target is small or the sound waves are of low frequency, sufficient backward scattering waves may not be obtained at the time of receiving the backward scattering waves. Therefore, there is a possibility of becoming incapable of searching the target accurately with the searching method which uses the backward scattering waves.
Further, in the case with a small target, the level of the forward scattering waves is normally higher than the level of the backward scattering waves. When receiving the forward scattering waves, there are also traveling waves other than the forward scattering waves, which travel towards the wave-receiving side from the sound source existing in an area distant from the target. The forward scattering waves are generated when the sound waves from the sound source make incident on the target and scatter therefrom. Therefore, the forward scattering waves may be hidden in the traveling waves which directly travel from the sound source, so that the target may not be searched accurately by using the forward scattering waves in some cases.